Electrically conducting layer



i May 20, 1952 K. B. BLODGETT ELECTRICALLY CONDUCTING LAYER 2 SHEETS-SHEET l Original Filed Jan. 29, 1947 v. S (959240159 asd 99%) f @o ...Q4/ly ob/fm1 tB/w nBt ee M Vn .H L Kw K. B. BLODGETT ELECTRICALLY CONDUCTING LAYER May 2o, 195,2y

2 SHEETS-SHEET 2 Original Filed Jan. 29, 1947 s TEMP. l/v ase. ff)

PROV/DED WITH SURF/46E LA YER OFREDUCE LEAD GLASS.

Inventor: KatharneBBlocLgett,

His Attorney.

Patented May 20, 1952 ELECTRICALLY CONDUCTING 'LAYER Katharine B. Blodgett, Schenectady, N. Y., assignor to `General Electric Company, a corporation of New York Continuation 'of abandoned application Serial No. l724,961, January '29, 1947, This application March 30, 1949, Serial No.184g466 1li-Claims. l

My invention relates to relectrical resistors, electrical insulators, electrical discharge devices, and other devices upon which for various reasons a conducting layer of predetermined high resistivity is desired.

This application is -a continuation of my copending patent application, Serial No. 724,961, filed January 29, 1947, now abandoned.

In applications wherein high voltages .are'employed, electrically conducting layers having high resistivities `and exhibiting stable characteristics are of great value. For example, a conducting layer of high resistivity which will not break down at vveryhigh voltage stresses maybe utilizedlas aresistive element between thegrid and cathode lof a discharge device, thereby permitting thessuccessful employment of very high control voltages. In high voltage discharge devices having insulating envelopes enclosing spaced operating electrodes, a conducting layer of high resistivity may lbe 'applied to the surface of the envelope to dissipate the electrical charges which tend to collect thereon Vand to distribute evenly the potential gradient between the electrodes, thereby permitting the utilization of very high `operating voltages with smaller electrode spacings. In a like manner, such conducting layers may be applied to high voltage transformer Vbushings and transmission line insulators to reduce permissible line togrcund and interline spacings'withoutincreasing -arcing possibilities.

Ithas been proposed heretofore to increase conductivity, or reduce the resistivity, of glassy mate rialsrby the application of highly conducting sub- 'stancessuch asm'etals to the surfacesor into the bodies of glasses. In such a manner, electronic conduction rather' than ionic is obtained.v and, subsequently, the polarization inherently accompanying the latter is avoided. Surface 'treatments for obtaining highly conductive layers have included metallizing procedures wherein salts of such metals as platinum, gold o-r-copper are heated -to a reduction on the glass, as well as many other methods involving chemical treatments. In all these procedures, however, control'of the conductivityof the applied layer has not been feasible since the insulating nature of the oxide `surface predominates until suicient metal has been applied to give the surface the high co-nductivity of the metal itself. This, lof course, dictates that the surface will be insulating sov long as the metal particles are not in contact .and when they are, conduction will be .in .the metalflayer' and will .dependsolelyon its llpr'operties. Thus, Vitlnas not been possible/by dictable, highsurface resistivity.

My invention has for its general object the production 1of .articles having conducting surface layers which exhibit high resistivities. It is another .object oi my invention to provide articles which .have conducting surface layers. the resistivity of which may be controlled and predictably produced within desired values. It is a further object of my invention to produce arti- -cles lhaving conducting 4surface layers which exhibit Vohmic characteristics and kare stable even under. very high voltage stresses.

AccordingV Vto one important feature of my invention, I may produce the aforesaid'conducting layers upon glass articles having lead oxide as a constituent material by the .simple expedient of heating specimens of such glass in a reducing atmosphere at certain specified .temperatures. To increase the predictability of the resistivity of the conducting layers, I .form a thin silica lm on the surface of the specimens before the heat treatment.

According to another important feature of my invention, I may Vproduce vconducting .suniace layers upon composite articlescomprising a suitable -foundation and a coating or coatings of glass kand Fig. .5 Vis a conventionalized representation of a .high fvoltage discharge device illustrative of vdevices upon which my electrically conducting layers may `be advantageously applied.

According to a first important feature of my invention, Iselect a suitable glass of high lead oxide content upon which aconducting layer is to be produced. Lhave'found that the lead glass commonly used in X-ray apparatus' ior'shielding purposes and referred to as X-ray shield glass possesses a `composition which is lsuitable forfemployin'g my'inven'tion. This glass ordinarily Iis Vformed.ironiabout 61% lead oxide, .8% barium oxide and 31% silica by weight. I have found'that higher contents of lead oxide accompanied bycorrespondingly lower contents ofsilica may be advantageously employed for certain purposes but the lower limit of the leadv oxide content isv about 60%,*aswi1l appear hereinafter.

`After selecting va suitable specimen of the l which may be of any size and shape according to" thepurp'osefor which it-will .be

subsequently used, I form a thin lm of silica on the surface thereof. One convenient way of obtaining this surface film comprises treating the specimen in a dilute solvent for lead oxide such as 0.01 HC1 at a temperature of 25 to 60 C. In this manner, lead oxide may be leached out of the glass specimen to a depth which is determined by the strength and temperature of the acid and the length of time of treatment. The rate of leaching increases rapidly with solvent temperature and a film of approximately 1,000 A. U. thickness may be obtained in about three minutes when the solvent temperature is approximately 35 C. This nlm may also be obtained by evaporating lead oxide from the surface of the specimen, butin such event care must be taken to avoid exceeding the melting point of the specimen.

After forming the aforesaid silica film, I next heat the specimen in a reducing gas such as hydrogen at predetermined temperatures for Specifled lengths of time. Since the silica film is porous, the hot hydrogen will readily penetrate the film and reduce the lead oxide in the specimen to form a surface layer beneath the silica film. Providing this treatment in hydrogen is carried out in a proper manner as described hereinafter, the reduced layer will have a desired predictable high resistivity.

Fig. 1 shows the time of treatment of specimens .-r

` Each curve represents thechange in resistivity of a given specimen as it is heated in hydrogen at the constant temperature with which the curve is labeled. It will be noted that in each case the surface resistivity of the specimen decreases from an initial, approximately infinite, value fairly rapidly at first and then more slowly to approach asymptotically a lower limiting value. The curve for 480 C. reaches a lower limiting value of about 1.l 105 megohms per square very quickly, and this value will not vary appreciably, no matter how long the time of treatment is extended. Treatment at 500 C. will impart some surface conductivity (the inverse of resistivity) to a specimen, the lower limiting value being about 8X105 megohms per square. However, if a specimen is heated at 520 C., no appreciable conductivity will be imparted to a specimen at all. The curve for 335 C. has not yet reached its lower limiting value after 8 hours treatment, but extension of the treatment time shows that such a lower limiting value will be reached. Extrapolation of the curves between 375 C. and 400 C. shows that the lowest limiting value of resistivity will be reached within this range of temperature.

Although the theoretical description of this lphenomenon is not fully understood, I have evolved the following explanation based upon a nucleation theory. In processes where nuclei occur, it is commonly observed that'large numspecimens, therefore, it may be assumed that within the low temperature region of the range 1n which nuclei appear, say from 315 C. to

400 C., a large number of very small nuclei are formed. As the treatment is extended, these nuclei grow to become very small spheres which eventually touch each other to form connecting chains. When the flrst connecting chains are formed, the specimen begins to obtain conductivity. Further extension of the hydrogen treatment increases the number of chains and hence increases the conductivity, or decreases the resistivity.

In the high temperature region of the range in which the nuclei appear, say from 400 C. to 500 C., a much smaller number of relatively large nuclei are formed. Extension of the treatment time in this event will cause the nuclei to form spheres as before, but a relatively few number of chains will be formed because the spheres remain isolated to a greater extent.

In the low temperature region where a great number of nuclei are formed, the decrease of resistivity to approach asymptotically a lower limiting value is apparently caused by the increase in thickness of the conductive layer as the treatment time is extended. The hydrogen will not penetrate the thicker conductive layer too readily to reach the surface of the specimen and further reduce the lead oxide therein. Therefore, the rate at which additional chains are formed decreases as the treatment time increases and an approximately constant lower value of resistivity is ultimately reached. In the upper region, however, the lower limiting value of resistivity would seem to be caused by the fact that only a small number of chains can be formed between the relatively large nuclei, and these are soon established. Subsequent extension of the treatment time, therefore, will not decrease the resistivity.

Curve A in Fig. 2 shows the end values of resistivity for each one of the curves in Fig. 1 after 7 hours treatment in hydrogen. It is to be observed that the lowest value of resistivity after 7 hours treatment occurs at approximately 400 C. Extension of the treatment time beyond 7 hours indicates that the lowest value of resistivity will occur between approximately 370 C. and 400 C. for the X-ray shield glass. Apparently, the reason for this is that the hydrogen will not penetrate the conductive layer to as great a depth at the lower temperatures and, therefore, even though greater numbers of nuclei are formed at the lower temperatures, the lower limiting value of resistivity will be higher. Above 400 C. the lower limiting value of resistivity will be determined by the number of nuclei formed rather than by the depth to which the hydrogen can penetrate the conductive layer. Therefore, between 370 C. and 400 C. the combination of the two factors, i. e., the depth to which the hydrogen will penetrate the conductive layer and the number of nuclei formed, results in the lowest limiting value of resistivity.

As is indicated by Fig. 1, the time at which conductivity is first imparted to the specimen invobserved that the time at which conductivity first appears, or the starting time where the 'curves strike the X-axis, varies from lessftha-n one-half hour at 400 C. to more than nine hours Asuits capable 'of 'easy'canalisation- Obviously, the

at 315 C. These'starting times .provide a convenient measure of the time at which thernuclei 'population is established'because-bythe time con- |duetivityiirstf appears, 'the-number of nuclei'must 'have 'been 'estab-lished andthe rst conducting vchains formed. Fig. 4 shows that'thesestarting 'timesall fall lapprtmim-ately on a straight line vwhen plotted on .a .logarithmic scale against the inverse of the treatment temperature in degrees Kelvin on ai linear scale. Obviously, the length of timev required'to reach'the starting time attempera-tureslbelow about 300 C. to 315U C. isso long thatlittle practical advantage vcan result from using temperatures below about 30.0 C.

As stated hereinbefore, the number of nuclei formed will be much greater in the low temperature .region than in 'the 'high .temperatureregiom vand therefore, more chains can befformed lWith the final limitation of the depth to which the hydrogen can penetrate the conductive llayer at fthe particular treatmentternperature. `However,

in the high temperature region, the chains will be formed` much more quickly. Consequently, if the number of nuclei is established within the Slow temperature region and the treatment temperature is then :raised to a high value such as l'500 or 52.0 C., amaximum niunber oi conductive chains may be formed within a minimum length -of time. This two temperature 'treatment 'may be advantageously employed to obtain lower final.

values of `resistivity, as well as to realize a sub- 'stantialsaving in time. Fig. 3 shows the various lengths of time at which a, specimen must. be heated at various temperatures to .exceed the starting time. reached -or exceeded, 'the temperature-may then 'beraisedtofa hgher'value.

I .have also. found it' convenient to obtain a conductive layerby inserting "a specimen into the hydrogena'tmcsphere at arlow temperatnrefwhich .may be .about 300 -C., and then to increase lthe temperature at. a. constant or known rate tto, a high `value not. exceeding l520 C. As. may be realized from theabove description, if kthe temperature is .raised at. a rate such that the integrated time spentiinthe low temperature region ywill cause the nuclei .to be fixedin this region,

'then a A:chai low value-of resistivity may be obtained. 1I have.found .that .fa ratefof 2 .Cz/minute providesgc'od .resultsfalthough Amuch slower'rates y `may obviously be employed. In alike manner, somewlfiat lfaster 'rates may he utilized, but the .integratedE times must be such that the nuclei are 'fixed in ithe vlow temperature region .in order vtoobtain a nalllow vvalueaof resistivity.. CurveB in'Fig. S2: vis a. plot fof resistivity aftentreatment vof'"shiel'dlglass specimens in hydrogen attemperatu-res Afrom 1336* C. to I `C`. "with .the temperature rising atzarateof 2 C./minute, T C'..be ing .the 'temperatureshown in thel abscissa. After the temperature of 'I' C is. reached, vthespecimens: may hev maintained at T C. for a given timefsuch as 30 minutes in. order to provide retime spent-.at 'I CL may be varied as'desired The conducting layers formed .upon vspecimens 'of' 'shield glass in this manner are ordinarily about 1Go-'20G A. U. in thickness. I have found tion will have a linear currenti-vol-tagecharacter- After the starting .time has been"l 'such layers to be :verystable under high voltage :stresses and to..e: :hibit ohmic characteristics A 'resistive element;produced-asserting'romy' inven- .6 istie. atfgradients upV 1:01100. kilovolitspenincnproviding resistivity is such as to vk'eeplthe energy dissipation below about 6.3 w'attperfsqu`are..inch. In "order to form electrodes 'upon 'a .specimen yto provide a resistive element, each fend `may' .be dipped into about a 5%. Vsolution.ofpolyvinylacetatev in acetone, 'such `.solution containing .flake graphite. This is-done fafter .the silica iiflm has been I applied but before .the hydrogenltreatment. Sincefthe silicalm is. relatively porous, it has low electricalresistance.

-It lis not 'essential for .imparting conductivity toftheshield glass'thatthe silica nimble zformed on the surface thereof .asv hereinbefore described. I prefer to utilise a silica nlm, because. in v'this manner I am able tosecure highlypredictable results, andthe curvesshown .in Fig. 11 may'ibe easily duplicated Within :narrow limits, providing the composition of the specimens remains `substantially the same. I have found that, ifa specimen remains in air for some time, a lvery thin uneven film of `silica. is formed on lits-surface, presumably as a result of acidic vaporsin the atmosphere. This nlm 'Will not 'prevent the formation of my conductive layers, of lcourse, but it lwill cause: thev obtained resistivities. Ito vary considerably from specimen to'specirnen.V Therefore, if a silica nlm of known thickness, 'or at least of-even thickness, is formedion `the'suriace of thee-hield' glass prior to the hydrogen treatment., the resistivities obtained will beuniform. The silica layer mayhave a convenientthickness such as 15G-5,000 A. U. I prefer to use att-hickness of about 1,080 A. U'. because such a .thickness vreflects an easily recognizedfblue interference color. A silica` krlhn which isv formed; by leaching lead from the surface of the specimen in a suitable solvent Willfatflrst be'soft, butafter the specimen has been heated at .from 400 "to 500 C. in air or hydrogen, it `will become lvery hard, thereby servingv additionallyas a .protective agent for the conductive layer formed underneath.

In order to further assure uniform'resistivities in my conductive layers, -I have `found 'thatannealing the specimen =to vremove possible 'strains may be advantageous. Also, the lower limiting value ofv resistivity, which is obtained aiter. hydrogen treatment, may 'be decreased somewhat if thezsurfaces offthe specimens are ground with awet carborund-um wheel priorto vthe'application of the silica lm., Apparently, this. increases the exposed surfacearea which thehydrogenimay reach,A thereby aidingv theformation of nuclei.

For .some Apurposes the `physical propertiesL Yof lead glass `may not be. Well situated for 'thef'fabricationof devicesupon which a surface modified by reduction isl desired. Therefore, accordance with another important feature of "my-invention, I apply a coating or coatings of lead .glass upon a selected foundationv and 'by' subsequent reductiom secure a'compo'site article havinga high, stable surface resistivity.

Infaccordance with this feature of'm-y' inven- 1tion,..I also4 selecta :glass having lead oxide. ias .a constituent material, e. g. .X-ray'shield tglass. As Ya. rst istep, .I :comminute and .finely powder theshield glass, for examplegto'aboutv an Bil-mesh zsize. Thereupon, I applythe nely dividedxglass as athin coating on the; surface of a .foundation glass vsuch as borosilicate glass, soda lime glass, lead glass with Vailovver lead oxide vcontent; or a: suitable.' ceramicv .material. .I .hare round'. .bcrcsilicate glass .te be. particularlysuited as a foundatlcnlnateria-l.:for: useivtithhi'sh voltage electron discharge devices and therefore, the following description will be concerned principally with such a glass as a foundation, even though other materials may be employed with suitable efficacy.

In applying the powdered glass to the foundation, I have found it convenient to suspend the powdered glass in a suitable liquid medium, e. g. a solution of nitrocellulose in amyl acetate or other vaporizable medium. This mixture may be applied as a thin coating on the surface of suitable specimen of borosilicate foundation glass as by flowing, dipping or spraying. A coating of about 0.5 mil thickness is satisfactory. After the foundation glass has been thus coated, it is heated, preferably in air, for about a half hour at a temperature at which the coating will vitrify and form a tight bond with the underlying glass, 630 for the borosilicate glass. After the article has cooled, the powdered glass coat will have the appearance of a shiny glaze and cannot be loosened from the foundation glass by the test of applying gummed tape against the coating and pulling it off suddenly. Following the application and vitrication of the rst coating, a second coating of the same powdered glass may be applied in identical fashion to the first. The heat treatment of this second coating, however, must be conducted at a somewhat lower temperature, say approximately 570 C. when X-ray shield glass is used as coating material. This temperature is high enough to make the second coating adhere to the first but low enough to prevent the second coating from being substantially incorporated by fusion into the rst. Therefore, the second coating can be considered as being sintered. It has a mat appearance after cooling as contrasted with the glaze on the rst coating. The thickness of the second coating should also be about 0.5 mil. Following the application and heating of the second coating, a silica film is formed thereon in the manner above described to insure predictable results.

To impart conductivity to the coated article so produced, it is baked in the presence of a reducing gas, such as hydrogen, at temperatures corresponding to those employed in connection with the X-ray shield glass described hereinbefore. As has been explained, these reducing temperatures are of critical importance, and the requirements mentioned with regard to temperature and time of treatment must be observed to secure effectively predictable results.

It is to be noted that the temperature at-which the second coating is heated in air before the hydrogen treatment should not be much above 570 C. If this coating is heated at temperatures ranging from 600 to 630 C., the resulting resistivity after hydrogen treatment is not readily predictable. The limits of this temperature are therefore determined on the one hand by the fact that the second coating must adhere to the first and on the other hand by the fact that the predictability of the resistance must not be destroyed. In many cases, it is advantageous that the rst coating should have substantially no conductivity after hydrogen treatment and, consequently, it is desirable to heat the first coating at as high a temperature as does not exceed the melting point of the underlying foundation. As will be explained hereinafter, the coating compositions may be varied within certain ranges, and in such event,the temperatures at which the coatings are heated may be advisedly varied t achieve the above purposes.

In the foregoing description I have mentioned several alternatives for the foundation material upon which coatings may be applied. The foundation material, of course, should have a substantially infinite electronic resistance as well as a suitably high resistance to ionic conduction. I have found that the ceramic material, such as porcelain, may be advantageously employed as a foundation for my coatings. The coatings may be applied to porcelain in a manner similar to that described above in connection with borosilicate glass. It may be desirable, however, to heat the first coating in air at a temperature of about 670 C. rather than 630 C., in order to cause the coating to adhere more securely to the porcelain.

While I have described in particular the application of two coatings upon a suitable foundation material, and the subsequent reduction in a reducing atmosphere to produce surface conductivity upon the outer coating, it may be desirable for some applications to apply only one coating to a foundation material. In such event. the foundation material must be such that the coating may be caused to adhere thereto without exceeding the temperature ranges above stated in connection with the application of the second coating. I have found that one coating may be applied to porcelain with suitable results, but I prefer to employ two coatings in order to enhance adherence to the porcelain. Therefore, while I deem it to be within the scope of my invention to apply only one coating, I find that the application of two coatings will secure more predictable and reliable results with porcelain as a foundation.

In making a resistive element from such coated articles, electrodes may be formed by painting a band of silver paint upon the first coating at both ends of the specimen. Thereafter, the second coating may be applied as hereinbefore described, and the conductive layer developed by hydrogen treatment. It is inadvisable to place the silver paint upon the second coating because the oil from the paint will be absorbed by the second coating and reduce the area upon which a conductive layer may be developed.

In Fig. 5 there is shown an electron discharge device illustrative of further application of my invention. Such an electron discharge device may be employed for the generation of X-rays or for any applications requiring the use of an electronic or ionic discharge by high applied voltages. The glass envelope I, which may be evacuated and consist of a suitable borosilicate glass, is coated either inside or outside or both inside and outside in the manner above described. Commonly, the inner surface of the envelope is coated with two superimposed coatings of shield glass. The window 5 may be left uncoated for the passage of X-rays to the exterior. The coatings 2 may each have a thickness of about 0.005. The reduction of the shield glass as described results in a conductive layer having a desired predetermined resistivity which is suitable for equalizing the distribution of charges which tend to accumulate on the glass envelope, as well as for distributing evenly the potential gradient which exists between the cathode 3 and the anode l when the device is in operation. An application, Serial No. 725,089, filed concurrently herewith by Ernest E. Charlton and now abandoned, describes and claims electronic devices in which are incorporated coatings having a conductive surface layer prepared in accordance with the present invention.-

While the above description has lbeen concannedv with.` Xeray shield Vglass havingl Vapproxi-- mately the: stated composition, I havefoundthat higher contents ofy lead oxide, e. g. up to 85%- by weight, may-be advantageously'employedfor the, production; ci` my conductive layers. `Asftl'ie` leady oxide content .is increased; the-curves Fig. `1v will.l :beshifted` corresp.ondinglywith;` thel lowest limiting value of resistivityloccurring within a lower temperature range than 375 C. Yto400." C. If the lead oxide content decreased lbelow about 60% by Weight, predictable resistivities 'cannot be ,obtained. I thereforeprefer to" employ coin-.-v positionsfwhich have a lead oxide content:1r-,ana-A ingbetween y60%. and 8.5%.

Whattlf. claim as new and desire to securebyb LettersaPatent `ofthe Unitedzstateslis:

l.. 'Ihemethod'of imparting apredictablehigh; surface resistivity to lead glass which comprises;

forming. a thinsuriace film. of silicaa-on-:a specimen lci lead glass having: a. lead; oxide; content:

of: at: least. l60% by.' Weight., and; heating specimen inlalreduoingatmosphere'at a-substan-` tiaily .constant temperature selected` from v.the rangeoi about.300 C. to 500 C.,fora.timezcorrelated; with. the selected temperature.

2; 'Ihemethod of impartinggapredictahlehigh surface-resistivity to; lead glass which-comprises; forming a thinY surface lm of: silica .Ona speci;- msnof lead; glass-'having alead 4oxidecontent of. atleast-6.0% by weight,..andheating.said-specie men in a reducing `atmosphere `at` a rising'temperature having a known rate of rise to produce a conductive layer beneath said film, the upper limit of said temperature being about 520 C. and the rate'of rise not exceeding a value at which measurable surface conductivity will be imparted to said specimen at a temperature beloW about 400 C.

3. The method` of imparting a high surface resistivity to lead glass which comprises heating a specimen of lead glass having a lead, oxide content of at least 60% by weight-,in areducing atmosphere first at a temperature betweeny about 300 C. and-400 C. until a measurably conductive layer isproduced upon the surface-of said specimen andl then raising the temperature;Y above about1400 C. but not exceeding about 5205 C; to impart l.further conductivity.r to said layer.

4. The method of imparting ar high surface resistivity to lead glass which comprises forming a thin surface film of silica on a specimen of lead glass having a lead oxide content of at least 60% by weight, and heating said specimen in a reducing atmosphere first at a temperature between about 300 C. and 400 C. until a measurably conductive layer is produced upon the surface of said specimen beneath said iilm and then at a temperature above about 400 C. but not exceeding about 520 C. to impart further conductivity to said layer.

5. The method of fabricating a high resistance conductor which comprises applying to a foundation of insulating material at least one coating of lead glass having a lead oxide content of at least 60% by weight to produce a composite article, forming a thin silica film on the outer surface of said coated composite article, and heating said article in a reducing atmosphere at a substantially constant temperature selected from the range of about 300 C. to 500 C. for a predetermined time correlated with the selected temperature to form a conductive layer on the surface of said varticle beneath said film.

6. The method of fabricating a high resistance 10' conductor which comprises applying toaA foundaen-,material atleast one coating of lead glass having-1 av lead oxide content of at' least 60% by weight to produce-a composite article, forming a.

thin silicafilm onthe outer surface of thefcoated composite article, and heating the article, in a reducing vatmosphere at a temperatur-e .havinga known rate-of riseto produce-a conductive layer onthesurface of the; article beneath said film,

the; upper limit. of said temperature being about 520 C. .andthe rate of risenotexceedingavaluey dation of .insulating material at least .one` coat-V ingV lof-lead glass having a lead-'oxide content of at least y60% -by weight to produce-acomposite article; and heatingl said composite article in aA reducingetmosphere first at a temperaturebetween about 300 C.. and 400 -C. until ameasurably conductiyelayer .is produced upon... the surfaceof- .said` .article andy then raising the temperaturerabove ,about 40.0 C. but nottexceeding about 5205?- G. to impar-t further--conductivitytog said layer.

8'. Thefmethod ofl fabricating a; highresistance conductorwhich comprises `applying to a` foundation; of insulating.r material at leastone coating. of lead-glass having a.- lead oxide :content oitv at least 60% by weight. to f produce a-.composite article;l `forming; a thi-n silica :film onftheY outer surface; ofthe, coated. article,- and heating the articlein a; reducing latmosphere iirst atV a' temperaturefbetween: abcnt300 C. and-400 C. until: ameasurably conductive layerv is`4 pirniflicedV uponthe surfacefcf saidiarticlebeneatn said filmV and then: ata-,temperature aboyezabout 400 but not exceeding about,.520 toA impart further conductivity .t'otsaifslv layer.

9. The method op fabricatinga high resistance'- conductor which comprises. applyingupo-n the surface of:y an.v insulating. material; a `rst coating of powdered leadfglass having.` a lead' oxideacontent'of atleast60%, by weight, heatingfthe .coatedf material, at; a temperature sufficiently,- highf to.

bond the, lead glass to the, insulating *materialJ applying: to:V the; material. thus` coated: a .secondz coating'cfj saidapowdered lead glass, heatingat; a lower temperature.'whichV is fsuiicientlyf high-to sinter said second coating, forming a thin surface film of silica upon said second coating, and thereupon heating the coated material in a reducing atmosphere at a substantially constant temperature selected from the range of about 300 C. to 500 C. for a time correlated with the selected temperature.

10. The method of fabricating a high resistance conductor which comprises applying upon the surface of an insulating material a rst coating of powdered lead glass having a lead oxide content of at least 60% by weight, heating the coated material at a temperature suiciently high to bond the lead glass to the insulating material, applying to the material thus coated a second coating of said powdered lead glass, heating at a lower temperature which is suiiiciently high to sinter said second coating, forming a thin surface lm of silica upon said second coating, and thereupon heating the coated material in a reducing atmosphere at a rising temperature having a known rate of rise to produce a conductive layer on said second coating beneath said silica lm, the upper limit of said last-mentioned 11l temperature being about 520 C. and the rate of rise not exceeding a value at which measurable conductivity will be imparted to said layer at a temperature below about 400 C.

l1. The method of fabricating a high resistance conductor which comprises applying upon the surface of an insulating material a rst coating of powdered lead glass having a lead oxide content of at least 60% by weight, heating the coated material at a temperature suillciently high to bond the lead glass to the insulating material, applying tof the material thus coated a second coating ofsaid powdered lead glass, heating at a lower temperature which is suiliciently high to sinter said second coating, and thereupon heating the coated material in a reducing atmosphere first at a. temperature between about 300 C. and 400 C. until a measurably conductive layer is produced upon the surface of said second coating and then raising the temperature above about 400 C. Vbut not exceeding 520 C. to impart further conductivity to said layer.

12. The method of fabricating a high resistance conductor which comprises applying upon the surface of an insulating material a rst coating of powdered lead glass having a lead oxide content of at least 60% by weight, heating the coated material at a temperature suiiiciently high to bond the lead glass to the insulating material, applying to the material thus coated a second coating of said powdered lead glass, heating at a lower temperature which is sufciently high to sinter said second coating, forming a thin surface lm of silica upon said second coating, and thereupon heating the coated material in a reducing atmosphere first at a temperature between about 300 C. and 400 C. until a measurably conductive layer is produced upon the surface of said second coating beneath said silica film and then raising the temperature above about 400 C. but not exceeding 520 C. to impart further conductivity to said layer.

13. A high resistance conductor comprising a specimen of lead glass having a lead oxide content of at least 60% by weight, said specimen being supercially freed of lead to form a thin lm of silica on the surface thereof, and a thin electrically conductive layer of reduced lead oxide formed on the surface of said specimen beneath said iilm, said layer having been produced by the method of claim 2.

14. A high resistance conductor comprising the combination of a foundation of insulating material and a coating thereon of glass having a lead oxide contentof at least by Weight, said coating being superiicially freed of lead to form a thin lm of silica on the surface thereof and having a thin electrically conductive layer of reduced lead oxide formed beneath said film, said layer having been produced by the method of claim 8.

15. A high resistance conductor comprising the combination of a foundation of borosilicate glass and a plurality of coatings of lead glass thereon having a lead oxide content of at least 60% by weight; the outermost coating being in a sintered, unglazed condition, having a thin illm of silica formed on the surface thereof, and containing a reduction product produced by the method of claim 8.

16. A composite article consisting of a foundation of borosilicate glass and successive superimpose coatings on said foundation consisting of lead glass having lead oxide content of 60 to by weight and a thickness of about onehalf mil; the first coating adjacent the foundation being glazed and shiny, the second coating superimposed on said rst being sintered and unglazed and having a mat appearance, said second coating having a thin film of silica formed on the surface thereof and containing a reduction product beneath said iilm which has stable ohmic characteristics under high voltage stresses.

KATHARINE B. BLODGETT.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 344,415 Shirley June 29, 1886 2,025,099 Gelstharp Dec. 24, 1935 2,027,413 Andres Jan. 14, 1936 2,220,862 Blodgett Nov. 5, 1940 2,251,144 Lytle July 29, 1941 2,314,804 Willson Mar. 23, 1943 2,339,928 Hood Jan. 25, 1944 2,369,741 J ones, et al Feb. 20, 1945 2,457,678 Jira Dec. 28. 1948 

1. THE METHOD OF IMPARTING A PREDICTABLE HIGH SURFACE RESISTIVITY TO LEAD GLASS WHICH COMPRISES FORMING A THIN SURFACE FILM OF SILICA ON A SPECIMEN OF LEAD GLASS HAVING A LEAD OXIDE CONTENT OF AT LEAST 60% BY WEIGHT, AND HEATING SAID SPECIMEN IN A REDUCING ATMOSPHERE AT A SUBSTANTIALLY CONSTANT TEMPERATURE SELECTED FROM THE RANGE OF ABOUT 300* C. TO 500* C. FOR A TIME CORRELATED WITH THE SELECTED TEMPERATURE. 